Lithium Bioleaching is a process that involves the extraction of lithium from a source material, such as lithium-containing ores or spent lithium-ion batteries (LIBs), using microorganisms. The process can be divided into two main mechanisms: direct and indirect.
Direct Bioleaching Mechanism
In direct bioleaching, microorganisms physically come into contact with solid materials, such as mineral particles or lithium-containing compounds. The key steps in the direct bioleaching of lithium are as follows:
Attachment of Microorganisms: Microorganisms, such as bacteria, attach themselves to the surface of the solid material. The attachment is often facilitated by extracellular polymeric substances (EPS) secreted by microorganisms containing polysaccharides, proteins, and lipids.
Microbial Oxidation: The microorganisms initiate oxidation reactions, typically involving sulfide and reduced metals in the solid material. For example, they can oxidize sulfur compounds and transfer electrons from the solid material to the attached cells.
Extracellular Polymeric Substances (EPS): EPS also play an essential role in the bioleaching process, especially at the interface of the mineral solution. They help with cell adhesion to the metal sulfide surface, creating a biofilm where bioleaching reactions occur.
Electrostatic Interactions: Electrostatic forces, mainly due to the positive charge on the cell surface created by chelated Fe3+, aid in the adhesion of microorganisms to the solid surface.
Proteomics Technology: Proteomics-related research has shown that protein-related molecules within EPS can accumulate sulfur, contributing to subsequent reduction reactions and the production of sulfuric acid.
Indirect Bioleaching Mechanism
In indirect bioleaching, microorganisms do not directly contact the solid substrates. Instead, they produce oxidants and reductants, such as organic and inorganic acids, responsible for dissolving the metals. Sulfuric acid is a commonly used inorganic acid in the indirect mechanism, while organic acids like citric acid and gluconic acid can also be employed.
Here are the key reactions involved in indirect bioleaching:
Oxidation and Reduction Reactions: In indirect bioleaching, oxidants like ferric ion (Fe3+) are reduced by oxidation of ferrous iron (Fe2+) during the dissolution of metal sulfides. Complexing agents can chelate metal ions, forming stable complexes.
Enhanced Efficiency: The efficiency of the indirect method can be improved by increasing the concentration of leaching agents (e.g., H2SO4) on the solid surface through microbial biofilm formation.
Fig. 1 Direct Bioleaching vs Indirect Bioleaching [1]
